Creep behavior and degradation of creep properties of high-temperature materials are phenomena of major practical relevance, often limiting the lives of components and structures designed to operate for long periods under stress at elevated temperatures. In ultra supercritical steam turbines operating at temperatures up to 600° C., the rotors, airfoils, rotor casing, valve chest and other ancillary components are generally composed of high Cr (9-12%) martensitic steel. High Cr (9-12%) martensitic steel are generally comprised of iron with between 9-12 weight % (wt. %) chromium, relatively low carbon contents, and additions of alloying elements such as molybdenum, tungsten, cobalt, vanadium, niobium, nitrogen, and others.
Creep is a time-dependent deformation of a material under an applied lead which most often occurs at elevated temperature. Physical models of creep behavior in creep-resistant steels such as high Cr (9-12%) martensitic steel rely on a stable microstructure built from martensitic laths and organized into blocks, packets and prior austentite grains and stabilized by M23C6 carbides, which assume resistance to increments in creep strain depending on barriers to dislocation movement through climb and glide, where a dense dislocation network, fine particle dispersions, and elastic strain fields in the matrix are generally considered to present effective barriers to dislocation movement. Changes in the effectiveness of these barriers lead to changes in creep behavior of materials. Structural changes in the material usually accelerate the creep, which in turn accelerates the rate of appearance of intergranular creep damage. Creep terminates in rupture when unabated and has a significant impact on component lifetime.
In high Cr (9-12%) martensitic steels, structural changes usually accelerate the creep, which in turn accelerates the rate of appearance of intergranular creep damage. Correspondingly, microstructural stability during service is of great importance. The structural features generally expected to exert an influence on the creep rupture properties in high Cr (9-12%) martensitic steels include the martensitic lath and associated hierarchical structure (blocks and packets), the prior austenite grain structure, the formation of a three-dimensional network of M23C6 carbides associated with such structures, the dislocation density within the martensite laths, the polygonization conditions of the subgrains, the presence of fine, uniformly dispersed MX, nitrides, and carbonitrides within the martensitic lath structure, and the solid solution strengthening of the martensitic lath matrix by elements such as molybdenum, tungsten and cobalt. The high Cr (9-12%) martensitic steels are typically strengthened by a combination of matrix solute strengthening agents such as cobalt, molybdenum, and tungsten, matrix transformation to martensite with heat treatments, and MX matrix precipitate and M23C6 particle formation on prior austenite/packet/block/lath/subgrain boundaries. Creep strength is generally achieved by a high dislocation density and the combination of frictional drag on dislocations from solute in the matrix and also MX precipitate formation impeding dislocation movement as discrete obstacles during creep. As is understood, the term MX particle denotes a nitride or carbonitride particle such as (Nb, V)(N, C), where M represents metal atoms such as titanium, niobium, tantalum, zirconium, hafnium, and others, and X represents interstitial atoms. The conditions under which different metal atoms form MX particles vary with the compositions or steel alloys. Similarly, as is understood, M23C6 indicates a carbide such as (Cr, FE, Mo, W)23C6, where M is the carbide forming element such as chromium, iron, molybdenum, tungsten, and others. The M23C6 carbides primarily form at prior austenite boundaries, packet, block and lath boundaries while MX carbonitrides precipitate in the ferrite matrix, typically during the tempering process.
The standard heat treatments for high Cr (9-12%) martensitic steels generally involve austenization and/or normalizing and tempering. The austenization is usually carried out at high temperatures above the Ac1 temperature in order to dissolve most carbides and nitrides and obtain a fully austenitic microstructure. After cooling to room temperature, the microstructure generally becomes martensitic, with a relatively high dislocation density. The martensite is a distorted tetragonal form of the ferrite bcc crystal structure. Normally air cooling of 9-12% Cr steels is sufficient for martensitic transformation, because the high levels of chromium retard the diffusion of carbon and mitigates the formation of ferrite. This is typically followed by tempering in order to recover ductility.
The creep resistant high temperature martensitic steels may be produced through both wrought or cast manufacturing methods. Wrought manufacturing primarily comprising the steps of alloy design, melt processing, homogenization, thermomechanical processing, and heat treatment. In cast manufacturing, the thermo-mechanical processing is largely eliminated, leaving limited ability to develop strength in the alloy body except through alloy design and heat treatment. Where previous alloys had not displayed a comparative strength in cast manufactured versus wrought, the present alloys show at least equal performance. The performance of the a cast alloy is illustrated in FIG. 1, which illustrates duplicate tests of cast CPJ-7 alloy in both equiaxed and columnar solidification zones of a cast ingot to show the relative property equivalence of the two solidification zones.
During an exemplary cast production of the alloy, once alloy formulation is established, the alloy is melted in a vacuum induction furnace with close attention to melt stock purity, the loading/timing of additions, and the tight control of minor elements such as C, N, Cu, and B. The newly melted ingot is then subjected to a computationally optimized homogenization heat treatment. One characteristic of this class of alloy is it starts to solidify as BCC up to about 80% with the remaining liquid solidifying with a matrix of FCC phase. Thus, in order to properly treat the homogenization of a 9% Cr steel matrix, the chemical segregation in both the BCC and FCC regions requires further treatment. This is done by determining the incremental solid chemistry from that of the liquid, regardless of what phase(s) are forming during solidification. Fortunately, during a standard heat treatment the structure of the matrix is FCC (which also gives a more conservative estimate of the homogenization since diffusion in the FCC phase is slower than that in the BCC phase). After each heat treatment increment, the incipient melt temperature is interrogated as before and the heat treatment temperature is adjusted to take account for and take advantage of the increased homogeneity, and thus the change in incipient melt temperature. To assess the degree of homogeneity of the alloy after the computationally designed homogenization cycle, the minimum and maximum values of each of each element of interest is compared to the element chemistry nominal value. It is preferred that the residual inhomogencity be less than 10% overall, with 5% being better and 1% being desirable. What is meant by this is demonstrated in the following example: if the nominal chemistry value for Cr is 9.75 by weight percent, and after the homogenization cycle the Cr is estimated to range from 9.58 to 9.74 or 98-100% of the aim (accounting for 99.1% of the solid). Thus, in this example, with respect to Cr, the homogenization of the CPJ-7 steel is deemed to be one of the best results possible. The preferred formulation for CPJ-7 cast steel is as follows (all values in weight percent):Fe-0.012B-0.15C-1.5Co-0.025Cu-9.75Cr-0.4Mn-1.25Mo-0.022N-0.05Nb-0.20Ni-0.1Si-0.2Ta-0.2V-0.50W
The CPJ-7 steel casting is homogenized suitably dependent upon the section size, preferred residual inhomogeneity (1% being the most desirable) and maximum furnace temperature capability as outlined in Table 1. Thus, for castings with a 4 inch section thickness, <1% residual inhomogeneity and furnace capability of above 1300° C., the castings were homogenized in the following manner: 1130° C. for 1 hour followed by 1250° C. for an additional 8 hours. The casting is then slow cooled to room temperature in the furnace. At this point the casting is austenitized at 1150° C. for 30 minutes, or an appropriate time commensurate with the as-cast article dimensions, followed by air cooling of the cast article to below the martensite start temperature at which time the cast article is tempered at 700° C. for 60 minutes (or at temperatures for times commensurate with the thickest dimensions of the article and its desired tensile properties) before cooling it in air to room temperature.
The intent of this approach was to have the 0.2% yield strength (YS) be higher than that of other commercial cast steels so that it would be more appropriate for the steam turbine rotor casing or main steam valve chest. Heat treatment during the post austenization tempering stage can be adjusted to either raise, or lower, the 0.2% YS depending upon the desired degree of ductility or impact toughness by changing either the tempering temperature and/or the number of tempering cycles.
With the necessary emphasis on creep behaviors at higher temperatures, heat resistant alloys providing improved creep performance provide obvious advantage. Correspondingly, presented here is a creep resistant alloy comprised of at least iron (Fe), chromium (Cr), molybdenum (Mo), carbon (C), manganese (Mn), silicon (Si), nickel (Ni), vanadium (V), niobium (Nb), nitrogen (N), tungsten (W), cobalt (Co), tantalum (Ta), boron (B), copper (Cu), and potentially additional elements. The overall composition strengthens the matrix with solute additions and precipitates, stabilizes the various grain and sub-grain structures using carbides, and engenders a high density of dislocations with thermo-mechanical processing (TMP) and heat treatment. The overall composition ameliorates sources of microstructural instability such as coarsening of M23C6carbides and MX precipitates, and mitigates or eliminates Laves and Z-phase formation by positional control of composition. In an embodiment, the creep resistant alloy exhibits significantly unproved high-temperature creep strength in the temperature environment of around 650° C. as compared to other creep resistant martensitic steels, such as COST FB2, COST E, and COST B2 steels.
Additional objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.